The present invention relates in general to apparatus for use in driving tunnels, trenches, galleries, adits and similar excavations collectively referred to for convenience throughout this specification and claims as "tunnels." More particularly, however, the invention is concerned with a shield for use in such apparatus.
It is well known to utilize a shield in tunnel driving operations which shield employs a series of drive members disposed side-by-side in parallel configuration and supported on a frame. These members are then driven forwards by shifting rams and the frame thereafter drawn up. In order to create a permanent tunnel lining supporting the tunnel wall behind the shield it is also known to provide the drive members with rear extensions which define a rear shield into which lining elements can be placed prior to supporting the wall of the tunnel exposed when the drive members are advanced.
As the rear extensions or tails are moved up to follow their drive members a gap is inevitably left between the tunnel wall and the lining elements and partial collapse of the tunnel wall and/or settlement can occur. Also the lining elements can become inadequately supported and their stability suffers. Various measures can be adopted to avoid these problems, for example, a filling material such as fluid concrete can be injected into the gaps left by the advancing tails. High pressures are usually necessitated however and the filling material is thus prone to be forced into unwanted regions. Also where tunnel driving occurs in water-laden soil pressurized air is often used to keep out the leaking water and high losses of air occur from the rear shield.